Sensors, in particular gas sensors, have been utilized for many years in several industries (e.g., flues in factories, in furnaces and other enclosures, in exhaust streams such as flues, exhaust conduits, and the like, and in other areas). For example, the automotive industry has used exhaust gas sensors in automotive vehicles to sense the composition of exhaust gases, namely, oxygen. For example, a sensor is used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio for combustion.
One type of sensor uses an ionically conductive solid electrolyte between porous electrodes. For oxygen, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, i.e., reference gas, is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine's exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (“emf”) is developed between the electrodes according to the Nernst equation.
With the Nernst principle, chemical energy is converted into electromotive force. A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (“exhaust gas electrode”), and a porous electrode exposed to the partial pressure of a known gas (“reference electrode”). Sensors used in automotive applications typically use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine's exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor's ionic conductivity at low exhaust temperatures. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:
  E  =            (                        -          RT                          4          ⁢                                          ⁢          F                    )        ⁢                  ⁢    ln    ⁢                  ⁢          (                        P                      O            2                    ref                          P                      O            2                              )                      where:        E=electromotive force        R=universal gas constant        F=Faraday constant        T=absolute temperature of the gas        pO2ref=oxygen partial pressure of the reference gas        PO2=oxygen partial pressure of the exhaust gasDue to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (emf) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating in fuel-rich or fuel-lean conditions, without quantifying the actual air-to-fuel ratio of the exhaust mixture.        
Sensors often comprise a first electrode capable of sensing an unknown gas and a second electrode exposed to a reference gas, with an ionically conductive solid electrolyte disposed therebetween. Materials (contaminants), such as silicon (in forms such as, for example, silica (i.e., silicon dioxide)), lead, and the like, present in the unknown gas (e.g., engine exhaust), can poison or otherwise damage the sensing electrode. In order to prevent poisoning/damage to the sensing electrode, a protective layer can be applied to the sensing electrode. Protective layers can comprise spinels (i.e., magnesium aluminate) or metal oxides that have a high surface area for contaminants (e.g., silicon dioxide). The protective layer traps the contaminants, preventing them from reaching and poisoning the sensing electrode. There remains, however, a need for additional sensors, protective coatings, and methods for producing the sensors and protective coatings that reduce possible poisoning of the electrodes.